Many techniques are known to measure the depth as well as the azimuth of downhole assemblies deployed within a wellbore. The downhole assemblies may be a logging tool (used in wireline application) or a drilling assembly (used in drilling and logging while drilling applications) which comprise a plurality of sensors for measuring properties of the geological formation surrounding the wellbore.
Typically, in wireline application, the logging tool is connected to a surface equipment via a logging cable. The depth of the logging tool is determined by means of a calibrated measure wheel at the surface. The wheel has a known circumference and is rotated by the logging cable when the logging tool is run into the wellbore. The depth may be corrected by taking into account the stretch of the cable due to the weight of the cable in the wellbore, the weight of the logging tool and the history of the cable stretch characteristics change with usage.
Typically, in logging while drilling application, the drilling assembly is connected to a surface equipment via a drill string. The depth of the drilling assembly is determined by measuring the length of pipe that enters the well at surface. The depth may be corrected for the effects of drill string tension or compression.
During the deployment and operation of the logging tool and drilling assembly, these downhole assemblies may move erratically within the well bore (e.g. bouncing effects, sticking and releasing effects, friction, compression or tension of the pipe or cable). Thus, it is often difficult to estimate at a particular instant the precise depth of the downhole assembly. In addition, in logging while drilling application, an additional error is introduced by the lack of synchronization between the uphole and downhole clocks. As a consequence, log produced by the sensors of the downhole assembly will be incorrect as a result of the errors made when correlating measurements performed by the sensors of the downhole assembly with depth measurements made at the surface. Further, the aforementioned estimated depths will be insufficiently precise for high resolution measurements such as images.